#!/usr/bin/env python
#coding=utf-8
#author=xiaomayi0323
#date=2009/03/26
#filename=5-3

def mark(x):
    if 90 <= x <= 100:
        return "your's mark: A" 
    elif 80 <= x <= 89:
        return "your's mark: B"
    elif 70 <= x <= 79:
        return "your's mark: C"
    elif 60 <= x <= 69:
        return "your's mark: D"
    else :
        return "your's mark: F"
        
    
try:
    a = float(raw_input('enter you score:'))
except:
    print 'error'
else :
     print mark(a)